A switch-mode power converter is a power converter that produces an output voltage by using a switch network to switch energy-storage elements into different electrical configurations. Examples of energy-storage elements include inductors and capacitors. A switched-capacitor power converter is a type of switch-mode power converter that primarily utilizes capacitors to transfer energy. In such converters, the number of capacitors and switches increases as the conversion gain increases.
As used herein, “conversion gain” is either a voltage gain or a current gain. In those cases in which the switched-capacitor power converter produces an output voltage that is larger than its input voltage, the conversion gain is a voltage gain. On the other hand, in those cases in which the switched-capacitor power converter produces an output voltage that is smaller than the input voltage, the conversion gain is a current gain.
FIGS. 1 and 2 show two examples of switched-capacitor power converters that receive an input voltage VI from a voltage source 16 and provide an output voltage VO to a load 18. Both of the examples are also known as “cascade multipliers.” In FIG. 2, a number of the switching devices of the circuit in FIG. 1 have been replaced with series of multiple devices. This reducing the maximum voltage across individual devices in the circuit.
In normal operation, charging or discharging pump capacitors C1-C3 pumps charge along a chain of diode-connected NMOS transistors M0-M5. The phase voltages VP1, VP2 are one hundred and eighty degrees out of phase. Each of the NMOS transistors M0-M5 is diode-connected, thereby only permitting boost operation (i.e., VO greater than VI). Additionally, the efficiency is severely impacted because a significant amount of voltage is dropped across each of the transistors M0-M5 during normal operation. Therefore, there is a desire to operate the NMOS transistors M0-M5 in their ohmic region. But due to difficulty and/or complexity of driving the transistors M0-M5, a combination of both PMOS transistors and high-voltage transistors is typically used.
If the transistors in the switched capacitor power converter are integrated on a single substrate, it is desirable to use as few different types of devices as possible. This is because, for a given semiconductor process, cost is related to the number of mask layers. As the number of different types of devices in a semiconductor process increases so does the number of mask layers, and hence the cost.
Furthermore, it is well-known that electrons have a higher mobility than holes in silicon. Consequently, an NMOS device with a given on-resistance has a smaller gate capacitance than a PMOS device with the same on-resistance. It is also true that an NMOS device with a given gate capacitance has a smaller on-resistance than a PMOS device with the same gate capacitance. In a power converter, it is therefore desirable to replace as many PMOS devices in the main power path with NMOS devices and also to replace as many high-voltage devices with low-voltage devices.